Add test for finding scaling factors where one of the units is a standard unit.
Add test for finding scaling factors where one of the units is a standard unit.

cellml/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml,nickerso/libcellml,MichaelClerx/libcellml,cellml/libcellml,hsorby/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml,nickerso/libcellml,hsorby/libcellml